The present invention is broadly concerned with an improved method of manufacturing plastic containers with molded-in protective metal inserts, wherein the metal inserts are magnetically held in place relative to the mold during the molding process, while permitting automatic release of the inserts when the article is removed from the mold. More particularly, it is concerned with a method of molding plastic containers wherein metal inserts and a quantity of a plastic resin are loaded into a rotary mold equipped with magnets. The magnets secure the metal inserts in position during biaxial rotation and heating of the mold and release the inserts without damage when the finished container shrinks during cooling and/or is removed from the mold. Such molded-in metal inserts are particularly resistant to separation from the container and during use of the container effectively protect the container from excessive wear at the corners.
Rotary molded hollow plastic containers such as industrial waste containers, bins, buckets, storage tanks, drums, hoppers, trays and pans are used extensively. They are light weight, durable, impact resistant, corrosion free and are suitable for use in a wide range of temperature conditions. For these reasons, such containers are frequently used in the transportation industry. Airlines in particular, require containers for the collection of galley waste which are light weight, sized to fit the dimensions of the open spaces in the aircraft and which can endure rough handling by airline employees. Since such containers are often tilted and dragged across the tarmac for emptying, the corners are especially subject to wear. Durability of these containers can be enhanced without substantial additional weight by the addition of metal wear plugs or inserts at the outside corners and other areas of wear.
Although other methods such as vacuum forming or injection molding may be employed to manufacture containers of this type, rotational molding is particularly well adapted to the cost effective manufacture of large hollow containers, because rotational molding is a low pressure process capable of producing seamless large articles with generally uniform wall thickness, complex contours, molded-in graphics, a variety of molded-in colors and products with a good finish.
Rotary molding generally involves charging a hollow mold with a quantity of a liquid or powdered thermosetable plastic resin material, closing the mold and moving it into an oven wherein the mold is rotated while being heated. In particular, the mold is mounted on a spindle for biaxial rotation during the molding process. As the temperature of the mold and resin rises to the tack point of the resin, it adheres to the interior surface of the mold. Eventually, the resin flows together and evenly coats the interior surface of the mold. The mold is then moved into a cooling chamber, where rotation is continued as the mold is cooled, often by water spray or forced air. Shrinkage of the article within the mold normally occurs during cooling, and continued rotation of the mold serves to ensure that the shrinkage is uniform and does not result in distortion of the shape of the article, especially due to sagging.
Because the mold rotates about two axes from the time it is heated with the resin material therein until it is opened and the cooled article in a completed form is removed, any non-plastic insert to be molded into the article must be positioned in the mold and secured in place throughout the duration of the molding process. The insert must be secured with sufficient strength to withstand rotational forces which might cause it to be dislodged, but allows movement during shrinkage upon cooling and allows removal of the article from the mold.
Previous attempts to produce rotary molded articles with molded-in metal inserts employed threaded bolts or rods to secure the inserts to the mold during the molding process. The metal inserts were bored in order to receive threaded bolt fasteners. The mold itself was bored through to permit insertion of the bolts through from the outside of the mold for threading onto the inserts. Such methods have not proved to be satisfactory because failure to unscrew all of the bolt fasteners prior to opening of a mold could result in damage to the mold itself. In addition, such methods did not provide for release of the inserts upon shrinkage of the formed article during cooling. When shrinkage causes the article to pull away from the sides of the mold, the molded-in inserts strain against the bolts. Often such inserts are pulled out of the article before screws could be unscrewed from their fasteners and the article is cracked or otherwise damaged by the stress created.
Other manufacturing methods such as die casting and injection molding have employed magnets during a molding procedure. However, such applications do not employ magnets on the inner surface of a hollow mold to hold an insert in place during rotation of the mold.
U.S. Pat. No. 4,264,016 describes a drum assembly having preformed plastic inserts.
U.S. Pat. No. 3,961,013 describes a process for manufacturing pipe fittings by injection molding. A core containing a magnet is inserted into the mold to maintain a ring in place.
U.S. Pat. No. 3,910,748 describes a plastic mold construction in which magnets hold sections of the mold in place until they are otherwise attached.
U.S. Pat. No. 3,439,731 describes a die casting process using a magnetic core to position a tube.
The present invention overcomes problems previously outlined and that exist in the prior art, while providing a greatly improved method of manufacturing plastic articles or containers with molded-in metal inserts that are held in place during the initial molding procedure by magnets, but that are released by the magnet when forces are exerted by shrinkage of the container or during removal of the container from the mold. In particular, the method may be employed to secure or hold metal inserts in a selected position relative to the interior of a rotary mold, so that the inserts are automatically released from the mold when the article shrinks and/or is removed from the mold. The method includes providing a mold with at least one magnet located to be near the interior surface of the mold, placing a metal insert in the mold so as to be held in place by the magnet, charging the mold with a quantity of a thermoplastic resin, biaxially rotating and heating the mold to melt and distribute the resin evenly on the interior surface of the mold, cooling the rotating mold and thereafter removing a finished container with a molded-in metal insert therein.
Preferably, the mold is mounted on a spindle arrangement, which allows the mold to rotate simultaneously about a first axis, as well as a second axis which is perpendicular to the first axis. The magnets preferably extend into and present a concave curved inward facing surface to the interior of the mold, which facing surface is a smooth continuation of the mold interior surface and which mates with a convex curved outer surface of a metal insert.
The shank of each insert also preferably includes a circumferential groove which is filled with resin during the molding such that the resin in the groove solidifies in a tongue within groove relationship with the insert and the remainder of the container to help hold the insert in place.
The principal objects and advantages of the method of the present invention include: providing a method for molding protective metal inserts into plastic containers which method provides for holding the inserts in a desired position in the mold during a rotational molding process; providing such a method which secures the metal inserts to the mold during heating and allows release of the inserts from the magnets due to forces created during cooling, while rotating about two different axes; providing such a method which does not require the use of threaded bolts to hold the inserts to the mold; providing such a method which easily releases the metal inserts upon removal of the finished article from the mold or during shrinkage of the article upon cooling without worker intervention; providing such a method which employs magnets to secure the metal inserts to the mold; providing such a method which employs an elongate metal insert having a curvate outer facing surface; providing such a method which is suitable for use with a powder, prilled powder, micropellet or liquid synthetic resin material; providing such a method which is simple, efficient and economical to operate, which effectively produces a plastic container having molded-in metal inserts and which is particularly well-adapted for its intended purposes.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.
The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.